Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution caused by existing gasoline and diesel vehicles using fossil fuel. As a result, applications using the secondary battery are being increased owing to advantages thereof, and the secondary battery is expected to be applied to more applications and products.
Secondary batteries have different structures depending upon outputs and capacities required by applications and products, to which the secondary batteries are applied. For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDAs), digital cameras, and laptop computers, use small-sized light cells for reduction in the size and weight of the corresponding products. On the other hand, medium- or large-sized devices, such as electric bicycles, electric motorcycles, electric vehicles, and hybrid electric vehicles, use a medium- or large-sized battery module (or medium- or large-sized battery pack) having a plurality of cells electrically connected with each other because high output and large capacity are necessary for the medium- or large-sized devices. The size and weight of the battery module is directly related to the receiving space and output of the corresponding medium- or large-sized device. For this reason, manufacturers are trying to manufacture small-sized light battery modules.
Generally, a medium- or large-sized secondary battery module is manufactured by mounting a plurality of unit cells in a case (housing) having a predetermined size and electrically connecting the unit cells. A rectangular secondary cell or a pouch-shaped secondary cell, which can be stacked with high integration, is normally used as the unit cell. Preferably, the pouch-shaped cell is used as the unit cell since the pouch-shaped cell is light and inexpensive.
The secondary battery module is manufactured by electrically connecting the plurality of unit cells with each other. Especially, all or some of the unit cells are connected in series with each other to provide high output. As a result, an engineer may be exposed to a high voltage during the manufacturing of the battery module, and therefore, a great care must be taken for safety in manufacturing of the battery module. Electrical short-circuits during the assembly of the battery module can cause injury to the engineer and reduction in the performance of the unit cells. Consequently, more careful assembly of the battery module is required so as to solve the above-mentioned short-circuit problem, which greatly reduces the productivity of battery modules.
Also, it is necessary to interrupt the operation of the battery module in the abnormal state, for example, when overcurrent or overheating occurs, in order to secure the safety of the battery module. In addition, it is preferable that such interruption of the operation of the battery module be accomplished by module-type members so that the unit cells causing the abnormal operation can be checked and replaced.
Meanwhile, when the unit cells each has the same capacity and voltage, the medium- or large-sized secondary battery module is optimally operated. Although the unit cells have the same capacity and voltage, however, the unit cells may have voltage difference due to the restrictions in manufacturing the unit cells, which are caused by various factors. Also, voltage difference between the unit cells may occur due to several factors during the use of the battery module as well as during the manufacturing of the battery module.
Consequently, there is a high need to minimize the voltage difference between the unit cells, and thus, to optimally maintain the operation of the battery module.